Decimate multiple objects at once
Start by selecting all the objects you want to have the same modifiers applied to.
Add the decimate modifier to the active object. The active object is the one that is highlighted in a lighter colour than the others, it is also the one that you can alter in the properties window.
Press CtrlL and select modifiers. This copies the modifiers from the active object to all other selected objects.




If you want more control there is an addon called "Copy Attributes Menu" that you can enable (it is included with blender) that allows you to copy selected modifiers.

If you then want to apply the modifier to all objects you can use AltC and select Mesh from Curve/Meta/Surf/Text, it may sound odd to convert a mesh to a mesh but it applies all modifiers to the selected objects.